1. Field of the Invention
The present invention relates to a method for predicting an amount of dimensional accuracy defect which is caused when a metal sheet such as a thin steel sheet, an aluminum sheet or the like which is mainly applied to an automobile body is formed by a press, and more particularly to a method which can preliminarily simply and accurately predict an amount of dimensional accuracy defect (mainly, a wall warp amount, an angular change amount or the like) of a formed product caused by an elastic recovery after removing the product from a mold in a press forming.
2. Description of the Prior Art
Many parts of an automobile body are generally constituted by parts which are produced by press-forming thin steel sheets. However, in forming these parts by a press-forming, due to an elastic recovery behavior after the removal from mold (the removal of a formed product from a mold), the shape (dimension) of the formed product is changed from the designed value and there exists a case that drawbacks take place at the time of assembling or joining formed products. The joining is mainly a joining by a spot welding. These drawbacks are generally called dimensional accuracy defects. As such dimensional accuracy defects, the wall warp and the angular change or the like are known.
Recently, from the viewpoint of making the automobile body light-weighted and increasing the stability of the automobile, cases that thin steel sheets having a high strength or aluminum sheets being light-weighted but having a lower Young's modulus compared to the steel sheets are used as the material for the automobile body have been increasing and hence, the above-mentioned dimensional accuracy defects have been taken as crucial problems.
To solve such problems, as means for enhancing the dimensional accuracy, (A) the development of new techniques and (B) the adjustment by predictive techniques and the like have been mainly carried out. As a technique belonging to (A), inventors of the present invention have proposed a method which reduces a wall warp amount by applying a tension to wall portion by increasing the blank holding force at the final state of the press operation. As a technique belonging to (B), there has been known a method which predicts an elastic recovery amount after the removal of the sheet from a mold and determines the dimension of a mold with a value which is obtained by subtracting the predicted value of the elastic recovery amount from the target product dimension and then adjusts the dimension to the prescribed dimension after removing the sheet from the mold.
With respect to the above-mentioned two means, whichever means is adopted, it is extremely important to preliminarily accurately predict an amount of dimensional accuracy defect after forming based on information on a metal sheet to be formed, forming conditions and the like.
Heretofore, in predicting an amount of dimensional accuracy defect after press-forming a metal sheet, there have been known (1) a method which performs the prediction based on the experience of skilled workers, the stored past results and the like, (2) a method which performs the prediction based on a numerical simulations and the like. However, it has been pointed out that either one of these methods suffers from following problems.
With respect to the method (1), it is difficult for engineers having a little experience or a little storage of techniques to perform an accurate prediction and hence, the method (1) is not suitable for a practical operation. On the other hand, with respect to the method (2), although various techniques which are proposed in “Sosei to Kako” (for example, vol. 36, No.410 (1995) p203-210, vol.37, No. 410(1996) p1352-1336 and the like) have been known, the expertise such as numerical simulations, mathematics or the like is necessary and there also exists a problem that a simulation facility such as a computer becomes large-sized.